Variable Gain Amplifier

ABSTRACT

A method may include applying an input differential voltage to input terminals of an amplifier, a first input terminal coupled to a gate of a first transistor and a second input terminal coupled to a gate of a second transistor. The method may also include varying a gain of the amplifier by varying at least one of: a resistance of a first resistor, the first resistor coupled between a source of the first transistor and a source of the second transistor; and a resistance of a second resistor, the second resistor coupled between a source of a third transistor and a source of a fourth transistor; wherein: the third transistor is coupled at its drain to the drain of the first transistor; and the fourth transistor is coupled at its drain to the drain of the second transistor and a gate of the third transistor and coupled at its gate to the drain of the third transistor.

RELATED APPLICATION

This application is a Continuation of U.S. patent application Ser. No. 13/477,926 filed May 22, 2012, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to electrical circuits, and more particularly, to variable gain amplifiers.

BACKGROUND

An electronic amplifier is a device for increasing the power of a signal. In basic operation, an electronic amplifier uses energy from a power supply and controls its output to match an input signal shape but with a larger amplitude. Amplifiers are used in many electronics applications, including in communications, audio systems, servo motor controllers, and many other applications.

A key operational parameter of any amplifier is its gain. A gain is typically a multiplicative factor relating the magnitude of the output signal to the input signal. The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. In some applications, it is beneficial that an amplifier have a variable gain, wherein the gain of the amplifier may be modified or changed. However, existing approaches to adapting conventional amplifiers to provide for variable gain have disadvantages.

To illustrate, reference is made to FIG. 1, which depicts a conventional amplifier 100, as is known in the art. A conventional amplifier may comprise transistors 108 a, 108 b, drain resistors 112 a, 112 b, and current source 106. Each drain resistor 112 a, 112 b may be coupled between the drain terminal of a corresponding transistor 108 a, 108 b, respectively, and a high potential voltage source (e.g., V_(DD)). Each transistor 108 a, 108 b may in turn be coupled at its gate terminal to a differential input (e.g., V_(in) ⁺ or V_(in) ⁻) and at its source terminal to current source 106.

In operation, transistors 108 a, 108 b form a differential transistor pair to sense and track the input data represented by a differential input voltage V_(in), and producing a differential output voltage V_(out) at the drain terminals of transistors 108 a, 108 b. Those of skill in the art will appreciate that the gain A of amplifier 100 may be given by the equation A=g_(m1)·R_(D), where g_(m1) is the transconductance of each transistor 108 a, 108 b and R_(D) is the resistance of each resistor 112 a, 112 b. Thus, to make the gain A variable, one or both of g_(m1) and R_(D) must be made variable. For example, R_(D) may be variable if resistors 112 a, 112 b are replaced with variable resistors. As another example, those of skill in the art will appreciate that g_(m1) is a function of I_(SS), the tail current generated by current source 106. Accordingly, g_(m1) may be varied by varying I_(SS), which it turn varies the gain A. However, as those of skill in the art will appreciate, the common mode output voltage V_(cm, out) of amplifier 100 may be given by the equation V_(cm, out)=V_(DD)−I_(SS)·R_(D)/2. Thus, adjusting the gain by changing either of g_(m1) or R_(D) will lead to a change in the output common mode voltage. Changing the output common mode of amplifier 100 may be undesirable, as it disturbs the bias condition of the circuit.

SUMMARY

In accordance with some embodiments of the present disclosure, a method may include applying an input differential voltage to input terminals of an amplifier, the input terminals comprising a first input terminal and a second input terminal, the first input terminal coupled to a gate terminal of a first transistor and the second input terminal coupled to a gate terminal of a second transistor. The method may also include varying a gain of the amplifier by varying at least one of: a resistance of a first resistor, the first resistor coupled between a source terminal of the first transistor and a source terminal of the second transistor; and a resistance of a second resistor, the second resistor coupled between a source terminal of a third transistor and a source terminal of a fourth transistor; wherein: the third transistor is coupled at its drain terminal to the drain terminal of the first transistor; and the fourth transistor is coupled at its drain terminal to the drain terminal of the second transistor and a gate terminal of the third transistor and coupled at its gate terminal to the drain terminal of the third transistor.

One or more technical advantages of the present disclosure may be readily apparent to one skilled in the art from the figures, descriptions, and claims included herein.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and its features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a schematic diagram of an example conventional amplifier, as is known in the art; and

FIG. 2 illustrates a schematic diagram of a variable gain amplifier, in accordance with certain embodiments of the present disclosure.

DETAILED DESCRIPTION

The present disclosure may refer to the “size” of various types of transistors, including an n-type metal-oxide semiconductor field-effect transistor (NMOS). Unless otherwise specified, the description of a transistor's size, as used herein, describes the size parameter that affects the transconductance of the transistor. For example, for NMOS devices, “size” may refer to the width-to-length ratio of the gate and/or of the conducting channel of the device. Accordingly, a device that is described as being sized at a ratio as compared to another device may have a transconductance that is larger or smaller at that ratio as compared to the transconductance of the other device.

Further, the term “equivalent” may be used to describe two or more currents or two or more voltage potentials that may be designed to be approximately equal to each other. Though they may be designed to be approximately equal to each other, “equivalent” voltages, “equivalent” currents, or other “equivalent” items may include some variation due to factors including, but not limited to, device matching imperfections, semiconductor processing imperfections, and/or imbalanced operating conditions.

FIG. 2 illustrates a schematic diagram of an example variable gain amplifier, in accordance with certain embodiments of the present disclosure. Power may be supplied to variable gain amplifier 200 through a high potential power supply and a low potential power supply. For the purposes of this disclosure, a high potential power supply may be referred to as “V_(DD)”, and a low potential power supply may be referred to as “GND” or “V_(SS)”.

As shown in FIG. 2, variable gain amplifier 200 may comprise a first stage 202 and a second stage 204. First stage 202 may comprise transistors 208 a, 208 b, resistors 212 a, 212 b, variable resistor 216, and current sources 206 a, 206 b. Each resistor 212 a, 212 b may be coupled between the drain terminal of a corresponding transistor 208 a, 208 b, respectively, and a high potential voltage source (e.g., V_(DD)). Transistors 208 a, 208 b may in turn be respectively coupled at their gate terminals to opposite polarities of a differential input (e.g., V_(in) ⁺ or V_(in) ⁻). In addition, each transistor 208 a, 208 b may be coupled at its source terminal to a corresponding current source 206 a, 206 b, respectively. In some embodiments, transistors 208 a and 208 b may be of approximately the same size. Furthermore, variable resistor 216 may be coupled between the source terminals of transistors 208 a and 208 b.

Second stage 204 may comprise transistors 214 a, 214 b, variable resistor 218, and current sources 207 a, 207 b. Each of transistors 214 a, 214 b may be coupled at its drain terminal to the drain terminal of a corresponding first stage transistor 208 a, 208 b, respectively. Furthermore, each transistor 214 a, 214 b may be coupled at its source terminal to a corresponding current source 207 a, 207 b, respectively. In addition, transistors 214 a and 214 b may be cross-coupled such that the gate terminal of transistor 214 a is coupled to the drain terminal of transistor 214 b, and vice versa. In some embodiments, transistors 214 a and 214 b may be of approximately the same size. Variable resistor 218 may be coupled between the source terminals of transistors 214 a and 214 b.

A gain A of amplifier 200 may be given by A=G_(m1·R) _(L), where G_(m1) is the effective transconductance of first stage 202 as seen as seen at the output V_(out) of amplifier 200, and R_(L) is the effective load resistance as seen at the output V_(out) of amplifier 200. The effective transconductance G_(m1) may be given by:

G _(m1) =g _(m1)/(1+g _(m1) ·R _(S1)/2)

where g_(m1) is the transconductance of each transistor 208 a and 208 b and R_(S1) is the resistance of variable resistor 216.

The effective load resistance R_(L) may be given by:

R_(L) =R _(D)/(1−G _(m2) ·R _(D))

where R_(D) is the resistance of resistors 212 a, 212 b and G_(m2) is the effective transconductance of second stage 204 as seen at the output V_(out) of amplifier 200. The transconductance G_(m2) may be given by:

G _(m2) =g _(m2)/(1+g _(m2) ·R _(S2)/2)

where g_(m2) is the transconductance of each transistor 214 a and 214 b and R_(S2) is the resistance of variable resistor 218.

By variable substitution, the gain A may be given by:

$A = {\frac{g_{m\; 1}}{1 + {g_{m\; 1} \cdot \frac{R_{S\; 1}}{2}}} \cdot \frac{R_{D}}{1 - {\frac{g_{m\; 2}}{1 + {g_{m\; 2} \cdot \frac{R_{S\; 2}}{2}}} \cdot R_{D}}}}$

Accordingly, as seen by the above equations, negative resistance is added by second stage 204 in order to modulate the effective load resistance R_(L). In addition, so long as transistors 214 a, 214 b (corresponding to g_(m2)) and the resistance R_(S2) of variable resistor 218 are selected such that effective load resistance R_(L)>0, amplifier 200 will operate as an amplifier. Variable gain of amplifier 200 may be accomplished by modifying one or both of variable resistors 216 and 218, without affecting the common mode voltage of the output of amplifier 200 (which may be given by the equation V_(cm, out)=V_(DD)−I_(SS)·R_(D)/2, where V_(DD) is the supply voltage, I_(SS) is the tail current generated by each of current source 206 a, 206 b, and R_(D) is the resistance of each of resistor 212 a, 212 b).

Although FIG. 2 depicts current sources 206 a, 206 b, 207 a, and 207 b all generating tail currents equal to I_(SS), other embodiments in which currents generated by current sources 206 a and 206 b may be substantially different than those generated by current sources 207 a and 207 b.

As used herein, a current source (e.g., current source 206 a, 206b, 207 a, and/or 207 b) may include any electrical or electronic device configured to deliver or absorb electric current.

As used herein, a transistor (e.g., transistor 208 a, 208 b, 214 a, and/or 214 b) may comprise any system, device, or apparatus configured to control a signal at a pair of its terminals by a signal applied to another pair of its terminals. Although the FIGURES of this disclosure depicts transistors as n-type complementary metal-oxide-semiconductor field-effect transistors, transistors may include any other suitable type of transistor (e.g., n-type complementary metal-oxide-semiconductor field-effect transistors, bipolar junction transistors, junction-gate field effect transistors, insulated gate bipolar transistors, etc.).

As used herein, a resistor (e.g., resistor 212 a and/or 212 b) may comprise any electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law.

As used herein, a variable resistor (e.g., variable resistor 216 and/or 218) may comprise any electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law, wherein such proportion between voltage and current may be varied based on a control signal or other stimulus provided to the variable resistor.

All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment(s) of the present inventions have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A method, comprising: applying an input differential voltage to input terminals of a variable gain amplifier, the variable gain amplifier comprising a gain stage and an active load stage; and varying a gain of the variable gain amplifier without altering a common mode output voltage of the variable gain amplifier by varying an effective impedance provided by the active load stage to the gain stage.
 2. The method of claim 1, wherein varying the effective impedance includes source degeneration of the active load stage.
 3. The method of claim 2, wherein varying the gain includes source degeneration of the gain stage.
 4. The method of claim 1, wherein the effective impedance is negative.
 5. An amplifier, comprising: a gain stage; and an active load stage comprising: a first transistor directly coupled at its drain terminal to the gain stage and to a gate terminal of a second transistor; the second transistor directly coupled at its drain terminal to the gain stage and to a gate terminal of the first transistor; a first current source coupled to a source terminal of the first transistor; a second current source coupled to a source terminal of the second transistor; and a variable resistor coupled between the source terminal of the first transistor and the source terminal of the second transistor, wherein a gain of the amplifier is modified without altering a common mode output voltage of the amplifier by: varying an effective impedance provided by the active load stage to the gain stage.
 6. The amplifier of claim 5, wherein varying the effective impedance includes source degeneration of the active load stage.
 7. The amplifier of claim 6, wherein the gain of the amplifier is modified by source degeneration of the gain stage.
 8. The amplifier of claim 6, wherein the source degeneration of the active load stage includes varying a resistance of the variable resistor.
 9. The amplifier of claim 5, wherein the effective impedance is negative. 